CN109696415B - Gas absorption rate online measurement method based on fast Fourier transform - Google Patents

Gas absorption rate online measurement method based on fast Fourier transform Download PDF

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CN109696415B
CN109696415B CN201910036652.3A CN201910036652A CN109696415B CN 109696415 B CN109696415 B CN 109696415B CN 201910036652 A CN201910036652 A CN 201910036652A CN 109696415 B CN109696415 B CN 109696415B
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彭志敏
杜艳君
丁艳军
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Tsinghua University
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Abstract

An online gas absorption rate measuring method based on fast Fourier transform belongs to the technical field of Tunable Diode Laser Absorption Spectroscopy (TDLAS). The method adopts sine wave to modulate the output wavelength of a tunable semiconductor laser, extracts a characteristic frequency spectrum containing absorption information to reconstruct the transmitted light intensity by performing fast Fourier transform on the transmitted light intensity, and can effectively eliminate noise signal interference of other frequencies such as particles, light intensity fluctuation and the like; and establishing a relation between the laser light intensity and the wavelength by combining the intermediate variable eta, and fitting the reconstructed transmitted light intensity by taking eta as an independent variable to realize synchronous online measurement of the incident light intensity and the gas absorption rate. The method is simple and convenient to operate and wide in application range, solves the problems that the uncertainty of baseline fitting of the direct absorption method is large and the absorption rate cannot be accurately measured by the wavelength modulation method, and effectively improves the measurement precision of the gas absorption rate.

Description

Gas absorption rate online measurement method based on fast Fourier transform
Technical Field
The invention relates to an on-line gas absorption rate measuring method, in particular to an absorption rate measuring method based on a tunable diode laser absorption spectrum direct absorption method technology, and belongs to the technical field of laser spectrum and gas measurement.
Background
Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a laser measurement technique that scans molecular absorption rate with narrow-band laser to estimate parameters such as gas temperature and concentration, and has been developed gradually as one of the main means for gas parameter diagnosis due to its advantages of non-contact, high measurement sensitivity, and strong anti-interference capability.
The gas absorption rate is taken as a key core parameter of the TDLAS technology, and high-precision online measurement of the gas absorption rate is widely concerned and researched. At present, the gas absorption rate is generally measured by a direct absorption method (DAS) based on a triangle or sawtooth wave, the physical concept is clear, the operation is simple, but the following problems exist in the application: 1) the baseline (incident light intensity) fit uncertainty is large: the base line is generally fitted with a polynomial to form 'non-absorption areas' on two sides of the transmitted light intensity, and then the absorption rate is obtained according to the Beer-Lambert law and the laser frequency, but the non-absorption areas are difficult to obtain in actual measurement due to factors such as spectral line broadening, adjacent spectral line interference, limited laser wavelength scanning range and the like. The uncertainty of the selection of the non-absorption region may cause a large baseline fitting error, which may further cause an absorption rate measurement error, for example, a weak absorption with a peak value of 1%, and a peak value error of 100% may be caused by a baseline fitting error of 1%. 2) The triangular or sawtooth scanning frequency is low: the triangular or sawtooth wave is composed of a series of nf (n is 0,1,2 …) frequency multiplication sinusoidal signals, has higher requirements on the bandwidth of a measuring system, the scanning frequency of the triangular or sawtooth wave is generally dozens to thousands of Hz, and is not beneficial to reducing or eliminating vibration, particulate matters, laser fluctuation and other low-frequency noise interference in measurement. In addition, the sudden change of the current at the peak or the trough of the triangular or sawtooth wave causes the laser frequency to change dramatically, for example, the triangular wave needs to change the frequency change direction instantly, and the sawtooth wave needs to change from the maximum value to the minimum value instantly, which causes the laser frequency calibration at the sudden change to be difficult.
Compared with DAS, a wavelength modulation method (WMS) based on high-frequency sinusoidal modulation adopts a harmonic detection technology to infer information such as gas concentration, harmonic detection can eliminate the influence of a base line theoretically (harmonic signals in a non-absorption area are zero), greatly improves modulation frequency (can reach hundreds of kHz), and can reduce the interference of other frequency signals. At present, WMS based on harmonic detection has achieved a great research result in gas temperature and concentration measurement, but has less research on absorption rate measurement, spectral line parameter calibration, and the like, because: the harmonic signal is a multi-parameter coupling result, not only contains information of absorption rate (determined by gas temperature, pressure, component concentration, spectral line spectral constant and the like), but also is influenced by a plurality of factors such as laser performance parameters (such as light intensity, light intensity amplitude modulation coefficient, light intensity modulation and frequency modulation phase difference, frequency amplitude modulation coefficient and the like), a response coefficient of a detection system and the like, and the uncertainty of the parameters can cause the theoretical calculation value (or calibration value) of the harmonic signal to deviate from the true value, thereby causing the measurement error of the gas absorption rate.
Disclosure of Invention
The invention aims to solve the problems that the uncertainty of the baseline fitting of the direct absorption method in the TDLAS technology is large and the absorption rate cannot be accurately measured by the wavelength modulation method, and provides an absorption rate on-line measuring method based on fast Fourier transform analysis. The technical scheme of the invention is as follows:
1) a sinusoidal signal with the frequency omega is generated by a signal generator and is input to a laser controller for modulating the output current of the laser controller, and further modulating the wavelength of laser output by a tunable semiconductor laser at the center v of a spectral line to be measured0Nearby;
2) dividing laser generated by a tunable semiconductor laser into two paths through an optical fiber beam splitter, wherein one path of laser passes through a gas chamber to be measured after being collimated, and receiving transmitted light intensity through a first photoelectric detector; the other path of laser is injected into the interferometer, the emergent light intensity of the interferometer is detected through a second photoelectric detector, optical signals of the first photoelectric detector and the second photoelectric detector are collected through a collecting card and are converted into electric signals to be transmitted into a computer;
3) the instantaneous laser wavelength output by the tunable semiconductor laser is as follows:
Figure BDA0001946137330000021
wherein t is the detection time sequence, f is the modulation frequency,
Figure BDA0001946137330000022
at the central wavelength of the laser, a1、a2、a3Linear and non-linear wavelength modulation amplitudes respectively,
Figure BDA0001946137330000023
is based on the initial phase angle of the frequency multiplication,
Figure BDA0001946137330000024
And
Figure BDA0001946137330000025
the phase angles are frequency doubling and frequency tripling phase angles;
4) the instantaneous incident light intensity before the incident light enters the air chamber is as follows:
Figure BDA0001946137330000026
in the formula
Figure BDA0001946137330000027
Is the mean value of light intensity, i1For linear intensity modulation of amplitude, i2And i3At twice and three times the frequency amplitude, theta, respectively1Modulating the fundamental frequency phase difference, theta, for light intensity and frequency2And theta3The light intensity and frequency are modulated to obtain a second frequency multiplication phase difference and a third frequency multiplication phase difference respectively;
5) defining intermediate variables
Figure BDA0001946137330000028
Substituting it into formula (I) yields the relationship between the laser wavelength and the intermediate variable η:
Figure BDA0001946137330000029
wherein v (eta) is laser wavelength with eta as independent variable, "+/-" respectively represents frequency rising and falling edges;
6) substituting eta into formula (II), and obtaining the relation between the incident light intensity and the intermediate variable eta by using the following formula:
Figure BDA00019461373300000210
wherein, I0(η) is the incident light intensity with η as the argument,
Figure BDA00019461373300000211
B1＝i1cosθ1-3i3cosθ3,B2＝2i2cosθ2,B3＝4i3cosθ3,B4＝i1sinθ1-i3sinθ3,B5＝2i2sinθ2,B6＝4i3cosθ3；
7) the instantaneous transmitted light intensity through the gas cell is expanded into a Fourier series form:
Figure BDA00019461373300000212
wherein R is0Amplitude of the direct current term, RkAnd psikAnd substituting η into the formula (V) to obtain the relation between the transmitted light intensity and the intermediate variable η:
Figure BDA00019461373300000213
wherein, It(η) is the transmitted light intensity with η as the argument;
8) carrying out fast Fourier transform on the instantaneous transmitted light intensity received by the second photoelectric detector to obtain harmonic amplitude and initial phase angle at each frequency, and extracting direct current term amplitude R0And harmonic amplitudes R at kf frequencieskAnd an initial phase angle psikWhere k is 1,2 …, R0，RkAnd psikSubstituted into formula (VI), let η be [ -1,1 [ ]]The value is uniformly changed in the range, and the transmitted light intensity can be reconstructed; the method has the advantages that the interference of noise of other frequencies such as white noise, electromagnetism, vibration and the like is reduced by extracting the characteristic frequency spectrum containing the absorptivity information;
9) according to the beer-lambert law, the transmitted light intensity with eta as an independent variable and the incident light intensity with eta as an independent variable which pass through the gas chamber satisfy the relationship:
Figure BDA0001946137330000031
wherein α (v) is to be measuredThe gas absorption rate, a being the integrated area,
Figure BDA0001946137330000032
is a linear function of the spectral line to be measured;
10) substituting the formulas (III) and (IV) into the formula (VII), fitting the reconstructed transmitted light intensity in the step 8) by using η as an independent variable through the relational expression to obtain the incident light intensity I with η as an independent variable0(η), integral area A, and Gaussian line width gamma of spectral line to be measuredDLorentz line width gammaLAnd dick convergence β;
11) the Gauss line width gamma of the spectral line to be measuredDLorentz line width gammaLAnd substituting the dick convergence coefficient β into the corresponding spectral line function to calculate the linear function of the spectral line to be measured
Figure BDA0001946137330000034
Combining the integral area A obtained in the step 10), and utilizing a formula
Figure BDA0001946137330000033
The gas absorption rate to be measured α (v) is calculated.
In the above technical solution, the linear function of the spectral line to be measured is gaussian, lorentz, foait, rautia or Galatry function.
Compared with the prior art, the invention has the following advantages and prominent technical effects: the method combines the advantages of a direct absorption method, no calibration, direct measurement of absorption rate, strong anti-interference capability of a wavelength modulation method and the like, establishes the relation between laser frequency and intensity by utilizing an intermediate variable eta, extracts a characteristic frequency spectrum through fast Fourier transform to reconstruct transmitted light intensity, can effectively reduce the interference of noise of other frequencies such as white noise, electromagnetism, vibration and the like, has very high data processing repeatability, and can realize automatic software processing. The gas absorption rate can be used to further estimate information such as gas concentration, temperature, and spectral constant.
Drawings
Fig. 1 is a schematic diagram of the gas absorption rate measurement system of the present invention.
FIG. 2 shows the interferometer signals measured experimentally and the wavelength curves and residuals obtained by fitting the interferometer signals.
FIG. 3 is a transmission light intensity signal I measured by the experiment of the present inventiontAnd obtaining the harmonic amplitude and the initial phase angle at each frequency through fast Fourier transform.
FIG. 4 is the transmission intensity reconstructed by the present invention.
FIG. 5 is the incident light intensity I obtained by the inventive simultaneous fitting0And transmitted light intensity It。
Figure 6 is the absorbance and its residual measured by the present invention.
In the figure: 1-a signal generator; 2-a laser controller; 3-a tunable semiconductor laser; 4-fiber optic counters; 5-air chamber; 6-a first photodetector; 7-an interferometer; 8-a second photodetector; 9-acquisition card; 10-computer.
Detailed Description
The invention is described in further detail below with reference to the attached drawing figures:
fig. 1 is a schematic diagram of the gas absorption rate measurement system according to the present invention. The measuring system comprises a signal generator 1, a laser controller 2, a tunable semiconductor laser 3, an optical fiber beam splitter 4, an air chamber 5, a first photoelectric detector 6, an interferometer 7, a second photoelectric detector 8, a collecting card 9 and a computer 10. The signal generator 1 generates a sinusoidal signal with the frequency f, inputs the sinusoidal signal to the laser controller 2, and is used for modulating the output current of the laser controller 2, so as to modulate the wavelength of the laser output by the tunable semiconductor laser 3; laser generated by a tunable semiconductor laser 3 is divided into two paths by an optical fiber beam splitter 4, one path of laser passes through an air chamber 5 after being collimated, and transmitted light intensity is received by a first photoelectric detector 6; the other path of laser is emitted into the interferometer 7, the emergent light intensity of the interferometer 7 is detected by the second photoelectric detector 8, and the first and second photoelectric detectors convert optical signals into electric signals which are transmitted into the computer 10 after passing through the acquisition card 9.
Based on the measurement system, the invention provides an online measurement method for gas absorptivity of laser absorption spectrum based on fast Fourier transform, which comprises the following specific implementation steps:
1) selecting center wavelength v of gas spectral line to be measured from spectral database0A sinusoidal signal with the frequency f is generated by the signal generator 1 and input to the laser controller 2 for modulating the output current of the laser controller 2, and further modulating the wavelength of the laser output by the tunable semiconductor laser 3 at the center v of the spectral line to be measured0Nearby;
2) laser generated by a tunable semiconductor laser 3 is divided into two paths through an optical fiber beam splitter 4, one path of the laser passes through an air chamber 5 to be measured after being collimated, and transmitted light intensity is received through a first photoelectric detector 6; the other path of laser is emitted into an interferometer 7, the emergent light intensity of the interferometer 7 is detected through a second photoelectric detector 8, and optical signals are converted into electric signals by the first and second detectors, are collected through a collecting card 9 and then are transmitted into a computer 10; recording the peak time of the signal detected by the second photodetector 8 as the abscissa of the wavelength calibration point, sequentially defining the ordinate of each wavelength calibration point, i.e. the relative wavelength, as 1 × FSR, 2 × FSR, …, (n-1) × FSR, n × FSR, (n-1) × FSR …,2 × FSR, 1 × FSR according to the FSR value of the free spectral region of the interferometer used for experiments, wherein n is the number of peaks in a half period, and drawing each wavelength point under the time-relative wavelength coordinate, namely the instantaneous laser wavelength output by the tuned semiconductor laser 3 obtained by experimental measurement;
3) let the tunable semiconductor laser 3 output the instantaneous laser wavelength as:
Figure BDA0001946137330000041
wherein t is the detection time sequence, f is the modulation frequency,
Figure BDA0001946137330000042
at the central wavelength of the laser, a1、a2、a3Linear and non-linear wavelength modulation amplitudes respectively,
Figure BDA0001946137330000047
is based on the initial phase angle of the frequency multiplication,
Figure BDA0001946137330000048
And
Figure BDA0001946137330000049
the phase angles are frequency doubling and frequency tripling phase angles; fitting the instantaneous laser wavelength experimental signals obtained by measurement in the step 2) by utilizing a Matlab program to obtain each coefficient in the formula (I);
4) let the instantaneous incident light intensity before incidence on the gas cell 5 be:
Figure BDA0001946137330000043
in the formula
Figure BDA0001946137330000044
Is the mean value of light intensity, i1For linear intensity modulation of amplitude, i2And i3At twice and three times the frequency amplitude, theta, respectively1Modulating the fundamental frequency phase difference, theta, for light intensity and frequency2And theta3The light intensity and frequency are modulated to obtain a second frequency multiplication phase difference and a third frequency multiplication phase difference respectively;
5) to establish a correlation between light intensity and wavelength, an intermediate variable is defined
Figure BDA0001946137330000045
Substituting it into equation (I) yields the relationship between the laser wavelength and the intermediate variable η:
Figure BDA0001946137330000046
wherein v (eta) is laser wavelength with eta as independent variable, "+/-" respectively represents frequency rising and falling edges;
6) substituting η into formula (II), the relationship between the incident light intensity and the intermediate variable η can be obtained:
Figure BDA0001946137330000051
wherein, I0(η) is the incident light intensity with η as the argument,
Figure BDA0001946137330000052
B1＝i1cosθ1-3i3cosθ3,B2＝2i2cosθ2,B3＝4i3cosθ3,B4＝i1sinθ1-i3sinθ3,B5＝2i2sinθ2,B6＝4i3cosθ3；
7) the instantaneous transmitted light intensity through the gas cell 5 is also a periodic function of frequency f, which can be expanded into the form of a fourier series:
Figure BDA0001946137330000053
wherein R is0Is a DC amplitude, RkAnd psikThe relationship of transmitted intensity to intermediate variable η can be found by substituting η into equation (V):
Figure BDA0001946137330000054
wherein, It(η) is the transmitted light intensity with η as the argument;
8) carrying out fast Fourier transform on the instantaneous transmitted light intensity received by the second photoelectric detector to obtain harmonic amplitude and initial phase angle at each frequency, and extracting direct current term amplitude R0And harmonic amplitudes R at kf frequencieskAnd an initial phase angle psikWhere k is 1,2 …, R0，RkAnd psikSubstituted into formula (VI), let η be [ -1,1 [ ]]The value is uniformly changed in the range, and the transmission light intensity after reconstruction can be calculated;
9) according to the beer-Lambert law, the transmitted light intensity and the incident light intensity which pass through the air chamber satisfy the following relation:
Figure BDA0001946137330000055
wherein α (v) is the gas absorption rate to be measured, A is the integrated area,
Figure BDA0001946137330000058
for the linear function of the spectral line to be measured, according to the specific experimental conditions, the functions of Voigt, Rautian, Galatry and the like can be selected for representation, and the linear function is represented by the Gaussian line width gamma of the spectral line to be measuredDLorentz line width gammaLAnd dick convergence factor β;
10) substituting the formulas (III) and (IV) into the formula (VII), fitting the transmission light intensity reconstructed in the step 8) by using the FIT function in the Matlab program through the relational expression by using η as an independent variable to obtain the incident light intensity I with η as an independent variable0(η), integral area A, and Gaussian line width gamma of spectral line to be measuredDLorentz line width gammaLAnd dick convergence β;
11) the Gauss line width gamma of the spectral line to be measuredDLorentz line width gammaLAnd substituting the dick convergence coefficient β into the corresponding Voigt, Rautian or Galatry functions to calculate the linear function of the spectral line to be measured
Figure BDA0001946137330000057
Combining the integral area A obtained in the step 10), and utilizing a formula
Figure BDA0001946137330000056
The gas absorption rate to be measured α (v) is calculated.
Example (b):
1) example takes CO molecule as an example, measures its (2 ← 0) R (11) spectral line absorptivity, selects its central wavelength v from spectral database0＝4300.699cm-1(ii) a A sinusoidal signal with the frequency f being 1000Hz is generated by a signal generator and is input to the laser controller 2 for modulating the output current of the laser controller 2 and further modulating the wavelength v of the laser output by the tunable semiconductor laser 30Nearby;
2) laser generated by a tunable semiconductor laser 3 is divided into two paths by an optical fiber beam splitter 4, and one path passes throughAfter collimation, the light passes through a gas chamber 5 to be detected, and the transmitted light intensity is received by a first photoelectric detector 6; the other path of laser is emitted into an interferometer 7, the emergent light intensity of the interferometer 7 is detected through a second photoelectric detector 8, and optical signals are converted into electric signals by the first and second detectors, are collected through a collecting card 9 and then are transmitted into a computer 10; the time of each peak value of the interferometer signal obtained by the experiment is recorded as the abscissa of the wavelength calibration point, and the Free Spectral Range (FSR) of the selected interferometer is 0.05cm-1Sequentially defining the ordinate of each wavelength calibration point as 0.05,0.1,0.15, …,1.15,1.2,1.15, … 0.1.1 and 0.05, drawing each wavelength calibration point under the time-relative wavelength coordinate, as shown by 'o' in fig. 2, namely outputting the instantaneous laser wavelength of the tuned semiconductor laser 3 obtained by experimental measurement;
3) let the tunable semiconductor laser 3 output the laser instantaneous wavelength as:
Figure BDA0001946137330000061
wherein t is the detection time sequence, f is the modulation frequency,
Figure BDA0001946137330000062
at the central wavelength of the laser, a1、a2、a3Linear and non-linear wavelength modulation amplitudes respectively,
Figure BDA00019461373300000612
is based on the initial phase angle of the frequency multiplication,
Figure BDA00019461373300000615
And
Figure BDA00019461373300000616
the phase angles are frequency doubling and frequency tripling phase angles; for the measured instantaneous laser wavelength point shown in "o" of FIG. 2, fitting by Matlab program to obtain the laser wavelength point in formula (I)
Figure BDA0001946137330000063
a1、a2、a3、η、
Figure BDA00019461373300000613
And
Figure BDA00019461373300000614
the parameters are 0.4694cm respectively-1、0.4257cm-1、0.003135cm-1、0.0002535cm-10.9937 π, -0.9169 π and-0.9868 π. The fitting results are shown in fig. 2 as a black solid line, and the wavelength fitting residuals are shown in the lower part of fig. 2;
4) let the instantaneous incident light intensity before incidence on the gas cell 5 be:
Figure BDA0001946137330000064
in the formula
Figure BDA0001946137330000065
Is the mean value of light intensity, i1For linear intensity modulation of amplitude, i2And i3At twice and three times the frequency amplitude, theta, respectively1Modulating the fundamental frequency phase difference, theta, for light intensity and frequency2And theta3The light intensity and frequency are modulated to obtain a second frequency multiplication phase difference and a third frequency multiplication phase difference respectively;
5) to establish a correlation between light intensity and wavelength, an intermediate variable is defined
Figure BDA0001946137330000066
Substituting it into equation (I) yields the relationship between the laser wavelength and the intermediate variable η:
Figure BDA0001946137330000067
wherein v (eta) is laser wavelength with eta as independent variable, "+/-" respectively represents frequency rising and falling edges;
6) substituting η into formula (II), the relationship between the incident light intensity and the intermediate variable η can be obtained:
Figure BDA0001946137330000068
wherein, I0(η) is the incident light intensity with η as the argument,
Figure BDA0001946137330000069
B1＝i1cosθ1-3i3cosθ3,B2＝2i2cosθ2,B3＝4i3cosθ3,B4＝i1sinθ1-i3sinθ3,B5＝2i2sinθ2,B6＝4i3cosθ3；
7) the instantaneous transmitted light intensity through the gas cell 5 is also a periodic function of frequency f, which can be expanded into the form of a fourier series:
Figure BDA00019461373300000610
wherein R is0Amplitude of the direct current term, RkAnd psikAnd substituting η into the formula (V) to obtain the relation between the transmitted light intensity and the intermediate variable η:
Figure BDA00019461373300000611
wherein, It(η) is the transmitted light intensity with η as the argument;
8) performing fast fourier transform on the instantaneous transmitted light intensity received by the second photodetector, as shown in fig. 3, obtaining harmonic amplitude and initial phase angle at each frequency, and extracting dc term amplitude R0And harmonic amplitudes R at kf frequencieskAnd an initial phase angle psikWhere k is 1,2 …, R0，RkAnd psikSubstituted into formula (VI), let η be [ -1,1 [ ]]The value is uniformly changed in the range, and the transmission light intensity after reconstruction can be calculated, as shown in figure 4;
9) according to the beer-lambert law, the transmitted light intensity and the incident light intensity passing through the air chamber satisfy the relationship:
Figure BDA0001946137330000071
wherein α (v) is the gas absorption rate to be measured, A is the integrated area,
Figure BDA0001946137330000072
for the linear function of the spectral line to be measured, the Rautian function is adopted to represent:
Figure BDA0001946137330000073
from the measured spectral line Gauss line width gammaDLorentz line width gammaLThe linear function of the spectral line to be measured can be represented by Fuyitt (Voigt) and Galatry functions besides Rautian;
10) the transmission light intensity reconstructed in fig. 4 was fitted to the equation (VII) by substituting the equations (III) and (IV) into the equation (VII) using the FIT function in the Matlab program using η as an independent variable, and the fitting result is shown in fig. 5, where the integrated area a obtained by the fitting was 9.473 × 10-3cm-1Gauss line width gamma of spectral line to be measuredD＝5.03×10-3cm-1Lorentz linewidth γL＝5.614×10-2cm-1And dick convergence factor β is 0.02cm-1Where, according to integrated area a, experimental pressure (P101.2 kPa), linear intensity (6.436 × 10)-2cm-1The concentration of CO is 1.016% and the relative error with the concentration of CO (1.02%) is within 0.5%; at the same time, the Lorentz line width gamma is measured according to the experimentLReduced impact broadening coefficient 5.641 × 10-2cm-1Peratm (T299K) is also compared with the HITRAN2016 data database of 0.0564cm-1The/atm anastomosis is good, and the accuracy of the established gas absorption rate online measurement method based on the fast Fourier transform is proved.
11) Will be provided withGauss linewidth gamma of spectral line to be measuredDLorentz line width gammaLAnd substituting dick convergence coefficient β into Rautian function to calculate linear function of spectral line to be measured
Figure BDA0001946137330000074
Combining the integral area A obtained in the step 10), and utilizing a formula
Figure BDA0001946137330000075
The gas absorption rate to be measured α (v) is calculated as shown in fig. 6.

Claims (2)

1. An on-line measuring method of gas absorptivity based on fast Fourier transform is characterized by comprising the following steps:
1) a sinusoidal signal with the frequency omega is generated by the signal generator (1) and is input to the laser controller (2) for modulating the output current of the laser controller (2), and further modulating the wavelength of the laser output by the tunable semiconductor laser (3) to be in the center v of a spectral line to be measured0Nearby;
2) laser generated by a tunable semiconductor laser (3) is divided into two paths through an optical fiber beam splitter (4), one path of laser passes through an air chamber (5) after being collimated, and transmitted light intensity is received through a first photoelectric detector (6); the other path of laser is emitted into an interferometer (7), the emergent light intensity of the interferometer is detected through a second photoelectric detector (8), optical signals of the first photoelectric detector and the second photoelectric detector are collected through a collecting card (9), and are converted into electric signals to be transmitted into a computer (10);
3) the output laser instantaneous wavelength of the tunable semiconductor laser (3) is as follows:
Figure FDA0002536659210000011
in the formula: t is the detection time sequence, f is the modulation frequency,
Figure FDA0002536659210000012
at the central wavelength of the laser, a1For linear wavelength modulation of amplitude, a2And a3Are respectively non-linearModulating amplitude by using linear frequency doubling and frequency tripling wavelengths;
Figure FDA0002536659210000013
is based on the initial phase angle of the frequency multiplication,
Figure FDA0002536659210000014
And
Figure FDA0002536659210000015
respectively a frequency doubling phase angle and a frequency tripling phase angle;
4) the instantaneous incident light intensity before the incident light to the air chamber (5) is:
Figure FDA0002536659210000016
in the formula
Figure FDA0002536659210000017
Is the mean value of light intensity, i1For linear intensity modulation of amplitude, i2And i3At twice and three times the frequency amplitude, theta, respectively1Modulating the fundamental frequency phase difference, theta, for light intensity and frequency2And theta3The light intensity and frequency are modulated to obtain a second frequency multiplication phase difference and a third frequency multiplication phase difference respectively;
5) defining intermediate variables
Figure FDA0002536659210000018
Substituting it into formula (I) yields the relationship between the laser wavelength and the intermediate variable η:
Figure FDA0002536659210000019
wherein v (eta) is laser wavelength with eta as independent variable, "+/-" respectively represents frequency rising edge and falling edge;
6) substituting eta into formula (II), and obtaining the relation between the incident light intensity and the intermediate variable eta by using the following formula:
Figure FDA00025366592100000110
wherein, I0(η) is the incident light intensity with η as the argument,
Figure FDA00025366592100000111
B1＝i1cosθ1-3i3cosθ3，B2＝2i2cosθ2，B3＝4i3cosθ3，B4＝i1sinθ1-i3sinθ3，B5＝2i2sinθ2，B6＝4i3cosθ3；
7) the instantaneous transmitted light intensity through the gas cell (5) is expanded into a Fourier series form:
Figure FDA0002536659210000021
wherein R is0Amplitude of the direct current term, RkAnd psikAnd substituting η into the formula (V) to obtain the relation between the transmitted light intensity and the intermediate variable η:
Figure FDA0002536659210000022
wherein, It(η) is the transmitted light intensity with η as the argument;
8) carrying out fast Fourier transform on the instantaneous transmitted light intensity received by the second photoelectric detector (8) to obtain harmonic amplitude and initial phase angle at each frequency, and extracting direct current term amplitude R0And harmonic amplitudes R at kf frequencieskAnd an initial phase angle psikWherein k is 1,2, converting R0、RkAnd psikSubstituted into formula (VI), let η be [ -1,1 [ ]]The value is uniformly changed in the range, and the transmitted light intensity can be reconstructed;
9) according to the beer-Lambert law, the transmitted light intensity with eta as an independent variable and the incident light intensity with eta as an independent variable which pass through the gas chamber (5) satisfy the relation:
Figure FDA0002536659210000023
wherein α (v) is the gas absorption rate to be measured, A is the integrated area,
Figure FDA0002536659210000024
is a linear function of the spectral line to be measured;
10) substituting the formulas (III) and (IV) into the formula (VII), fitting the reconstructed transmitted light intensity in the step 8) by using η as an independent variable through the relational expression to obtain the incident light intensity I with η as an independent variable0(η), integral area A, and Gaussian line width gamma of spectral line to be measuredDLorentz line width gammaLAnd dick convergence β;
11) the Gauss line width gamma of the spectral line to be measuredDLorentz line width gammaLSubstituting the dick convergence coefficient β into the corresponding spectral line linear function to calculate the linear function of the spectral line to be measured
Figure FDA0002536659210000025
Combining the integral area A obtained in the step 10), and utilizing a formula
Figure FDA0002536659210000026
The absorption rate α (v) of the gas to be measured is calculated.
2. The fast fourier transform-based gas absorption rate on-line measuring method as claimed in claim 1, wherein: the linear function of the spectral line to be measured selects Gaussian, Lorentz, Foritet, Rautian or Galatry functions.
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CN110361359B (en) * 2019-07-16 2020-11-24 清华大学 Absorption rate function reproduction method based on even harmonic
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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7230711B1 (en) * 2002-02-08 2007-06-12 Southwest Sciences Incorporated Envelope functions for modulation spectroscopy
CN104931427A (en) * 2015-07-02 2015-09-23 中国科学院合肥物质科学研究院 Opto-acoustic gas detection device based on multiple reflections of optical path
CN107144549A (en) * 2017-05-11 2017-09-08 西安科技大学 Detection means and method based on TDLAS trace CO gas concentrations
CN107219190A (en) * 2017-05-05 2017-09-29 东南大学 A kind of non-calibrating gas parameter measuring method based on triangular modulation
CN207946353U (en) * 2018-03-29 2018-10-09 鞍山哈工激光科技有限公司 A kind of gas concentration detection apparatus
CN109085133A (en) * 2018-06-12 2018-12-25 东南大学 One kind being based on the modified off-axis integrated chamber atmosphere CH of real-time reflectivity4The measuring device and its measurement method of concentration

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7230711B1 (en) * 2002-02-08 2007-06-12 Southwest Sciences Incorporated Envelope functions for modulation spectroscopy
CN104931427A (en) * 2015-07-02 2015-09-23 中国科学院合肥物质科学研究院 Opto-acoustic gas detection device based on multiple reflections of optical path
CN107219190A (en) * 2017-05-05 2017-09-29 东南大学 A kind of non-calibrating gas parameter measuring method based on triangular modulation
CN107144549A (en) * 2017-05-11 2017-09-08 西安科技大学 Detection means and method based on TDLAS trace CO gas concentrations
CN207946353U (en) * 2018-03-29 2018-10-09 鞍山哈工激光科技有限公司 A kind of gas concentration detection apparatus
CN109085133A (en) * 2018-06-12 2018-12-25 东南大学 One kind being based on the modified off-axis integrated chamber atmosphere CH of real-time reflectivity4The measuring device and its measurement method of concentration

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Jidong Li 等."Measurements of spectroscopic parameters of CO2 transitions for Voigt, Rautian, Galatry and speed-dependent Voigt proÞles near 1.43μm using the WM-DAS method".《Journal of Quantitative Spectroscopy & Radiative Transfer》.2018,第224卷第197-205页. *

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